1. Field of the Invention
The present invention relates to optical waveguide couplers and more specifically to a low loss optical waveguide tee coupler.
2. Description of the Prior Art
Multimode optical data links generally employ two types of terminal interconnect components: the tee coupler which provides a tap to the terminal from a common bus; and a star coupler which functions as a power splitter to distribute optical signals to terminals coupled directly thereto. Both systems suffer from light loss brought about by the coupling elements. Light loss suffered in coupling two terminals of a multi-terminal system wherein the terminals of the system are coupled by means of a star coupler varies linearly with the total number of interconnected terminals, while the worst case light loss suffered in coupling two terminals of a multi-terminal system, wherein the terminals of the system are coupled by means of a bus and a multiplicity of tee couplers employing three mixer rods, varies geometrically with the total number of interconnected terminals. Although the beneficial loss properties of star couplers are important for systems containing a larger number of terminals, the relative advantage becomes marginal for optical data links having a small number of terminals. Moreover, star couplers require the use of more cable than tee couplers thus introducing additional light loss and significantly increasing the cost of the over-all system. More recent prior art tee couplers have reduced the number of mixing rods from 3 to 1 with a concomitant decrease in the three port coupling loss of 2db. The improvement being realized by the utilization of only one mixer rod to equalize the energy distribution among the fibers of the bus.
The defect in all tee couplers is the inherent through put loss which results when mixer rods are utilized to equalize the distribution of energy among the fibers of the bus. Because the through put loss mounts geometrically as the number of tee connectors in the link increases, the through put loss represents the single most critical parameter of tee couplers. Tee couplers exhibit a minimum insertion loss, even with negligible coupling to the side arm, due to packing fraction losses in the tee coupler which result from the equalization of the optical signals across all the fibers in the trunk. The light loss characteristics of a multi-terminal system employing a common bus and tee couplers cannot be improved without eliminating these packing fraction losses while providing some means for equalizing the light energy distribution among the fibers of the bus.